In a conventional stapler or stitcher, a pre-formed staple, or wire concurrently formed into a U-shape, is provided. Two staple prongs defining the U-shape are driven through a stack of paper and into an opposing surface. The opposing surface and a staple driving surface compress the stack of paper while the staple prongs are driven into the stack. Once the prongs have pierced the paper to contact the opposing surface, channels in the opposing surface operate to fold the staple prongs, thus securely cinching the stack of paper. In such a stapler, the length of the prongs limits the thickness of the paper stack that can be so secured. If the stack is too thick, the prongs will not extend sufficiently through the paper to firmly cinch the stack. If the stack is too thin, the excessively long prongs will meet the channels of the opposing surface and fold without contacting the associated surface of the stack. The resulting staple will only loosely bind the sheets of paper, leading to chaffing and tearing of the stapled stack of sheets. Thus a disadvantage of conventional staplers is that varied stapling applications require different length staples, and often different staplers.
A further disadvantage to these staplers, particularly those designed for relatively thick stacks of paper, is that each staple prong must have sufficient axial rigidity to pierce the complete stack, regardless of the strength required to permanently bind the stapled stack. Further, in order to pierce thicker stacks of material, each staple must have a sharpened, or "chisel" point to ensure good piercing. Where a concurrently formed wire is used, forming a sharpened point may not be practical. Thus additional design constraints are placed upon the staples which have no bearing on the strength of the final attachment.
Accordingly, it is an object of the invention to provide a stapler which can operate on stacks of sheet material having widely varied thicknesses without any compromise in the strength of the stapled attachment. It is further desired that the invention operate with any staple adequate to maintain the final attachment, without the need for the axial strength and sharpness required by customary staples to pierce sheets of material.